The present invention relates generally to die casting processes and relates more particularly to an improved die warm-up process for use in the production of castings which include metallic inserts.
It is common practice in the manufacture of die cast articles to employ separately fabricated parts that are cast in and become an integral part of the casting. Such parts are aptly termed "inserts" and are usually employed to provide properties not otherwise obtainable with the cast metal. Inserts may be used, for example, to provide additional strength, hardness, abrasion resistance or other property to a particular region of a casting. Although a variety of materials may be used for inserts, ferrous metal inserts are commonly employed where increased strength or hardness is required.
Inserts often represent a substantial portion of the cost of the finished casting, either because of their size, complexity, and/or the materials of which they are fabricated. Should a casting be scrapped, the die cast metal can be remelted and cast again. However, the inserts usually cannot be recovered and thus may represent a substantial economic loss. Furthermore, there is risk of contamination of the melting furnace by the inserts when it is charged with scrapped castings.
A number of scrapped castings are produced during the start-up of a die casting machine. Since the proper operating temperature of the dies is achieved and maintained in large part by the heat retained from preceding casting cycles, a number of preliminary die warm-up casting cycles are required before the die is at its correct casting temperature. Proper die filling and such important casting characteristics as dimensional accuracy, strength and surface finish are dependent on the correct die temperature.
The number of casting cycles required to warm up the die will vary somewhat depending upon the sizes of the die and casting, the amount of preheating available and the nature of the casting alloy among other factors. However, at least four or five casting cycles are usually required to bring the die to the correct temperature and the so-called warm-up castings produced during this procedure must be scrapped because of their poor quality.
In those castings utilizing inserts, the inserts employed in the warm-up castings normally cannot be recovered. Although in some instances the warm-up castings can be made without the use of the inserts, in most cases the inserts are required, for example to prevent the castings from seizing on the die insert support.